Various types of positioning systems for determining a position based on radio signals are known in the art. For example, satellite navigation systems allow autonomous geospatial positioning with virtually global coverage. Global navigation satellite systems (GNSS) provide GNSS receivers with the capability to determine their location based on positioning signals transmitted from the GNSS satellites in terms of longitude, latitude, and altitude to within a few meters or even centimeters. GNSS based positioning has a wide range of applications including navigation and tracking and automatic positioning.
Generally, for determining its position, a GNSS receiver first determines distances to a plurality of GNSS satellites. Each individual distance measurement made by the receiver to a satellite located in a known orbit position traces the receiver on the surface of a spherical shell at the measured distance from the satellite. By taking several such measurements and determining an intersecting point of the spherical shells, a position fix can be generated. The distance measurements to the satellites are based on a time of flight measurement of positioning signals transmitted by the satellites to the receiver and thus the measurements depend on an exact timing. Normally, three distance measurements to three known satellite positions are sufficient to resolve a receiver position in space, however, with the receiver clock offset from satellite clock time being the fourth unknown in the equations, measurements on four satellites are needed to determine the position of the receiver.
The orbit position of the satellite may be determined based on a data message superimposed on a code that serves as a timing reference. The receiver can compare the time of broadcast at the satellite encoded in the transmission with the time of reception measured by an internal clock at the receiver, thereby measuring the time of flight to the satellite. GNSS systems provide satellite data messages that transmit a code with a timing reference, enabling a receiver to compare a successively delayed internal replica of this code with the received code from the satellite. By progressively delaying the local copy, the two signals become aligned in time. That delay is the time needed for the signal to reach the receiver, and from this the distance from the satellite can be calculated.
The Real-Time Kinematic (RTK) method was developed to provide greatly improved accuracy in position determination, suitable for use in surveying. RTK positioning performs measurements of the carrier phase of the satellite signals and makes estimates of the exact number of carrier frequency wavelengths (19.6 cm) to each satellite. The method is well-known in the GPS/GNSS positioning arts.
Various error sources affect the absolute positioning accuracy. As noted above, the exact time of flight of the signal from the satellite to the receiver station must be measured, which may be in the range of e.g. 0.06 seconds from a satellite directly above a receiver. In order to make the time measurements as accurate as possible, GNSS satellites generally include several atomic clocks providing a highly accurate time reference. However, even atomic clocks suffer from a certain time error that constitutes an error source in the measurements that has to be observed when desiring centimeter level accuracy. Other error sources include propagation delays introduced by the troposphere and ionosphere, errors in the satellite positions as described by the orbital parameters, and other relativistic effects, as are well known in the art.
To improve the accuracy of the estimation, the RTK method provides reference data on the same set of satellite observables from another source to a surveyor's receiver, referred to as a rover receiver. These reference station observables are relayed to the rover via ground based radio transmission, in order to enable the receiver to perform the double-differencing process that removes error contributions. See for example “GPS Satellite Surveying,” by Alfred Leick, John Wiley & Sons, ISBN-10: 0471306266, or “Global Positioning System: Signals, Measurements and Performance Second Edition,” by Pratap Misra and Per Enge, Ganga-Jamuna Press, ISBN 0-9709544-1-7.
These systems often require strict continuity of data delivery of the correction stream. Typically, the data is delivered on a second-by-second basis to provide continuity of the correction stream. Thus, an interruption of the correction stream can cause delays in performing the desired survey activity, as the interruption can necessitate a resynchronization of correction and data reception in the rover, sometimes taking many minutes where the rover cannot be moved. Further, the effects of the ionosphere and troposphere are not homogeneous over areas much larger than 60-100 km, thus limiting the range of applicability of the network of reference stations.